Horizons. I've stayed at both. Vistana 3 years ago and Horizons this past March. We loved the Vistana but you do risk getting an older, not so great unit. Our kids were also older than yours. The resort is beautiful but huge and the units vary widely.
Horizons is very, very, nice. No whirlpool tub in the master but a beautiful shower and a good size second bedroom with double beds. The villa had an open airy feel to it and the pool area is fabulous. My girs were 12 & 8 and even the 12 year old had a blast. It was made for little boys your kids age.
We had either #14 or #15, I can't remember but it was a brand new unit, very close to the pool (like right outside the door) and a big screen TV. I requested ahead of time as those were the two newest units and they accommodated us no problem.
It's a little farther to Disney but I think only about 15 minutes total time (depending on traffic of course) to get there.
You'll enjoy either resort but I'd go with Horizons for sure.
Jenn